Amplifying circuit for capacitive microphones



Sept. 21, 1965 G. BORE 3,207,848

AMPLIFYING CIRCUIT FOR GAPAGITIVE MICROPHONES Filed Jan. 12, 1962 2 Sheets-Sheet .1

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transmissiansfaclar Unitcd States Patent 3,207,848 AMPLIFYING (JIRCUIT FOR CAPACITIVE MICROPHONES Gerhart Bor, Berlin-Zehlendorf, Germany, assignor to Georg Neumann, Laboratorium fur Eiektroalrustik G.m.b.I-l., Berlin, Germany, a corporation of Germany Filed Jan. 12, 1962, Ser. No. 165,920 8 Claims. (Cl. 179-1) The present invention relates to an amplifying circuit for capacitive microphones.

The human hearing is extensively insensible for low frequency sound alternating pressures and for direct pressures, as they occur for instance by the wind. The lowermost frequency sound pressures occurring in nature can surpass the normal hearing sound by several orders. If the hearing for these components would have the same sensitivity as for the median frequency range, they could often disturb considerably the normal hearing process.

Contrary to the hearing, the measure of transmission factor of a good microphone should not have such drop within the range of the hearing frequencies and thu does not have the natural protection against over raised low frequency sound pressure components. In case of a music transmission of high quality, generally a low limit frequency of 30 to 40 Hz. is required for the transmission path, that means also for the microphone.

It has been found by experience that particularly in microphones, the swinging system of which is mass-controlled (for instance a dynamic pressure gradient microphone) or tuned at a medium pitch and damped by friction (for instance an electro-static gradient microphone) a frequency range which extends up to the low frequencies must be taken in stride with a strong wind and structureborn sound sensitivity of the microphone.

In order to remedy this situation the following known measures have been applied:

For instance an electrical high-pass, which is switched after, locks the over-raised low frequency components. This measure has, however, the drawback that any over loading of the microphone tube cannot be avoided thereby. A cover of wire or textile tress work has been used also as a cover called in most instances wind protector which causes equalizing of the lowermost frequency sound alternating pressures and airstreams around this cover, without attaching the microphone. A low frequency structure-born sound is eliminated therefrom by means of an elastic suspension of the microphone. These two measures have, however, the drawback that essentially larger dimensions have to be taken in stride and, in addition, a sufiiciently soft suspension of the microphone often cannot be carried out by operational means. It is also known in connection with condensator and piezoelectric microphones to reduce so much the input impedance of the microphone amplifier (which is often simultaneously the grid resistance of the microphone tube), so that the low frequency parts fall down preferably already at the inner resistance of the microphone capsule. By this expedient the self-noise level, particularly of the condenser microphone is appreciably worsened, since the greater the input impedance of the microphone amplifier at a given capsule capacity, to so much lower frequencies the resistance and grid current rustling is electrically bridged by the parallelly disposed capsule capacity.

The swinging system of the pressure gradient condenser microphone has been already les damped. By creating an acoustic by-pass effective only for lower frequencies, the driving pressure gradient is increased to the same extent in low frequencies, as the mechanical resistance of the swinging system increases. However, this measure cannot be driven too far at will, if the conventional microphone dimensions should be maintained.

The amplifying circuit, designed in accordance with the present invention, has the advantage that by means of a mixed positive and negative feedback at high pass filter characteristic is obtained. By this arrangement, in condenser and piezoelectric microphones low-frequency voltage components are appreciably weakened below a desirable limit frequency by the use of lighter and space saving construction elements, particularly without inclusion of inductivities already at the grid of the first amplifying tube, that means in the microphone amplifier, while simultaneously in the adjacent range above the limit frequency no reduction of the frequency range which is important for the quality of the transmission, takes place. Over-loading and the feared closing of the preliminary amplifying tube occurring due to the great membrane deviations occurring from wind-and structure-born, etc., i extensively avoided by this arrangement, a result which was not possible with the conventional high-pass filters disposed at the output terminals of the microphone preamplifier.

Since the feedbacked voltage is connected with the footpoint of the microphone capsule, the very high inputimpedance of the amplifying tube, is not subjected to any change, and the steps taken in accordance with the present invention have no worsening of the selfnoise level of the microphone.

A particular advantage of the new circuit resides also in the fact that the inner resistance measurable at the starting terminals of the amplifier is nearly constant in the entire transmission range down to the limit frequency and only below this frequency becomes smaller. Thefrequency run is thus not, as in other circuits for the lowfreqnency-cut, depending upon the load resistance.

With these and other objects in view, which will become apparent in the following detailed description, the present invention will be clearly understood in connection with the accompanying drawings, in which:

FIGURE 1 shows the circuit, by example, of a condenser microphone with a one-step pre-amplifier;

FIGS. 2 and 3 depict curves for the transmission factor of the pre-amplifier versus frequency, and in particular FIG. 2 shows the principal run and FIG. 3 shows the curves on a circuit disclosed in FIG. 1; and

FIG. 4 is the spacial arrangement of the construction elements required for the positive and negative feedback according to the present invention in a condenser microphone.

Referring now to the drawings, and in particular to FIG. 1, the microphone M is disposed as a generator on the grid of the tube R5, which receives its grid pretension over the grid resistance R,,. For the feeding of the polarization voltage required for the operation of the condenser microphone is provided the resistance R The mixed positive-and negative feedback from the plate of the microphone tube to the footpoint of the microphone capsule comprises, for instance, the threemembers chain with the series resistances R to R and the cross condensers C to C The grounded terminal of the capsule is not connected directly, but over the last cross condenser of the chain, namely the condenser C with the zero potential. The condenser C merely blocks the D.C.-potentials from each other.

The alternating appearing at the plate is knowingly phase-removed for relative to the grid connected The previously stated threeor more members chain is limited in its dimensions, such that its total phase shifting at a predetermined limit frequency, preferably at about 40 Hz., amounts again to 180. Jointly with the 180 phase shifting by the tube, a total value of 360 resuits, and, thereby, a positive feedback and an overraising in the frequency response of several db at 40 Hz. In case of a three-members chain, it is possible, for instance, to lay out each member for a phase rotation of about 60. The calculation takes place in similar manner as for a phase shifting generator. Such generator requires, however, in order to permit its swinging, an amplifying factor of the tube of 29 in the case of a threemembers chain and of 18.4 in case of a four-members chain, provided it is composed of members which are equal relative to each other. In the case of the microphone amplifier, this amplifying factor is retained smaller and still reduced to lower frequencies by means of a frequency dependent negative feedback path, so that no self-excitation can take place.

The second feedback path, which is superimposed to the first one, consists of the resistance R and the con denser C and causes, for itselfa frequency response of the transmission factor decreasing at low frequencies, which transmission factor corresponds with the curve A in FIG. 2. If now only the multi-members feedback path is in action, respectively, a frequency path results corresponding with the curve B, that means, for higher frequencies the chain by and by becomes non-penetratable and ineffective. At both sides of a predetermined limit frequency f preferably at about 40 Hz., a positive feedback is effective depending, as to its size, substantially upon the amplification of the tube :360 at f which feedback is reduced again at lower frequencies with an increasing phase shifting and becomes negative at phase angles 4; of more than 450.

The members of the two feedback channels are dimensioned such, that above the limit frequency of for instance 40 HZ., the reduction by the negative feedback (R C and the rise by the positive feedback (R C .R C are equalized and result in a horizontal run of the frequency response curve. Below the limit frequency f :4O Hz. both feedback channels cause a lowering of the curve and without requiring a filter including inductances the amplification drops sharply to very small values, in spite of the additional frequency-independent negative feedback (capacitive from the plate over the grid-anode capacity of the tube R6 on the capsule M and the condensers C, and C disposed in series with the latter) which is provided in addition to the two feedback channels. A superimposing of both negative and positive feedbacks, respectively, results thus with suitable dimensioning to a frequency path corresponding with the curce C.

The steepness of the flanks in a circuit of FIG. 1 can be driven rather high. It can, if this is desirable, be reduced at will by an additional frequency independent negative feedback.

The limit frequency of the total arrangement, as well as the limit frequency of the members R C can be made switchable in a simple manner. In the first case, the value of the condenser C and finally also one of the resistances R to R in the second case, the value of the resistance R can be changed stepwise or continuously.

For the transmission of music and language, particular importance is attributed to the second possibility, particularly in case of directional microphones, because such microphones have knowingly a considerable increase in sensitivity for low frequencies in case of close speaking into the microphone. For instance, the output voltage of the pressure gradient receiver increases for a speaking distance of 10 cm. at 40 Hz. to the tenfold, compared with the tension at 1000 Hz. and equal sound pressure.

Since the microphone is subjected in such close speaking anyway often with sound pressures of 100 b. and more, which sound pressures can work well the capsule, yet not always free of distortions in working the preamplifying tube, it is an advantage, if the mentioned preference of the lower frequencies are compensated already at the grid of the microphone tube in accordance with the present invention, thus the tube cannot be overloaded as easily.

In the circuit disclosed in FIG. 1, the switch S is generally closed during the close speaking of directional microphones of the mentioned type and have as a consequence a frequency path of the transmission factor which is linear in the narrow field.

With the switch S the low frequency cut off can be made ineffective, for instance, if in a microphone position which is sufficiently safe against sound disturbances, organ receptions are to be made in an increased frequency range up to below 30 Hz., as compared with the curve D in FIG. 2. By opening of the switch S the amplifying frequency path is linearized for instance up to 20 Hz.

Finally, also by choice of a greater value for R a desired amplifying rise to low frequencies up to the limit frequency f can be obtained in the same sense of the curve B (FIG. 2) and, for instance, can be used as compensation of a too low microphone transmission factor in the same range.

FIG. 3 shows measuring values which have been obtained by the circuit shown in FIG. 1. The curves A to D have been taken under the same margin conditions, as they there stated above for the curves shown in FIG. 2.

The curve E results if additionally a frequency independent negative feedback of about 3 db is introduced. The curve F is obtained, if the resistance R by parallel arrangement of the resistance R by means of the switch S is reduced to about one-third of its original value. With a frequency response in accordance with the curve F, for instance the low frequency-increase of a pressure gradient microphone, spoken to at a distance of about 35 cm. can be compensated for.

In the part showing of an open condenser microphone, disclosed for instance in FIG. 4, the reference numerals have the same meaning as those shown in the circuit of FIG. 1. Only, the structural elements, determining the frequency path, are shown on the printed circuit plate of the microphone preliminary amplifier and the output transmitter U likewise disposed on this plate is shown. All parts of the microphone, which are not essential for the present invention, have been deleted. The space requirements for the seven structural elements in the shown arrangement amounts to 30 by 24 by 6 mm. A high path for 40 Hz. in the conventional structure with coils and condensers cannot be disposed within the same space.

While I have disclosed one embodiment of the present invention, it is to be understood that this embodiment is given by example only and not in a limiting sense, the scope of the present invention being determined by the objects and the claims.

I claim: '1. An amplifying circuit for capacitive microphones, comprising an amplifying tube including an anode, a cathode and a control grid,

a microphone capsule having two terminals,

one of said terminals being connected with said grid of said amplifying tube and the other of said terminals being connected with at least one capacitor, the other electrode of which is grounded, a filter chain having at least three stages and connecting i the anode of said amplifying tube with the connec tion of said capacitor and said microphone capsule,

each of said stages consisting of one series resistor and 'one bridging condenser, the other electrode of which is grounded,

the last of said bridging condensers being identical with 1 said capacitor connecting said microphone capsule with ground,

the voltage occurring at said anode of said amplifying tube being fed back over said at least three-stage chain to the connection of said microphone capsule with said last condenser,

an additional resistor disposed in parallel with said plurality of resistors, and said additional resistor and said last condenser of said filter cha-in forming a negative feedback path. 2. The amplifying circuit, as set forth in claim 1, wherein said filter chain provides a total phase shifting of 180 at a given frequency. 3. The amplifying circuit, as set forth in claim 1, wherein a positive feedback is caused by said filter chain within -a limited frequency range, and said negative feedback increasing at low frequencies by means of said additional resistor, the cooperation of said feed'backs resulting in an amplifier frequency response of high-pass-filter characteristic. '4. The amplifying circuit, as set forth in claim 3, wherein said positive feedback surpasses by a larger value the feedback due to said additional resistor, and thereby, an increase of the frequency response results within -a limited frequency section with a definite cutoff frequency at its lower frequency end, and said limited frequency section has a total phase shifting of said filter chain between 90 and 270. 5. The amplifying circuit, as set forth in claim 1, wherein said negative feedback surpasses by a smaller value the feedback due to said additional resistor, and

thereby an additional reduction of the amplification and of voltages occurring at the grid at lower frequencies for the compensation of the lower frequencies rise. 6. The amplifying circuit, as set forth in claim 5, which includes means for varying the value of the elements of said feedback lines. 7. The amplifying circuit, as set forth in claim 1, which includes a switch for disconnecting each of said two feedback lines. '8. The amplifying circuit, as set forth in claim 1, which includes means for flattening the flank-steepness of the curves I of the transmission factor, and said flattening means comprising an additional frequency-independent negative feedback.

References Cited by the Examiner UNITED STATES PATENTS 2,3 87,845 10/45 Harry l79'-1 2,629,025 2/53 Roberts 330-97 2,869,068 1/59 Morcerf et a1. 33097 ROBERT H. ROSE, Primary Examiner.

WILLIAM C. COOPER, Examiner. 

1. AN AMPLIFYING CIRCUIT FOR CAPACTIVE MICROPHONES, COMPRISING AN AMPLIFYING TUBE INCLUDING AND ANODE, A CATHODE AND A CONTROL GRID, A MICROPHONE CAPSULE HAVING TWO TERMINALS, ONE OF SAID TERMINALS BEING CONNECTED WITH SAID GRID OF SAID AMPLIFYING TUBE AND THE OTHER OF SAID TERMINALS BEING CONNECTED WITH AT LEAST ONE CAPACITOR, THE OTHER ELECTRODE OF WHICH IS GROUNDED, A FILTER CHAIN HAVING AT LEAST THREE STAGES AND CONNECTING THE ANODE OF SAID AMPLIFYING TUBE WITH THE CONNECTION OF SAID CAPACITOR AND SAID MICROPHONE CAPSULE, EACH OF SAID STAGES CONSISTING OF ONE SERIES RESISTOR AND ONE BRIDGING CONDENSER, THE OTHER ELECTRODE OF WHICH IS GROUNDED, THE LAST OF SAID BRIDGING CONDENSERS BEING IDENTICAL WITH SAID CAPACITOR CONNECTING SAID MICROPHONE CAPSULE WITH GROUND, THE VOLTAGE OCCURRING AT SAID ANODE OF SAID AMPLIFYING TUBE BEING FED BACK OVER SAID AT LEAST THREE-STAGE CHAIN TO THE CONNECTION OF SAID MICROPHONE CAPSULE WITH SAID LAST CONDENSER, AN ADDITIONAL RESISTOR DISPOSED IN PARALLEL WITH SAID PLURALITY OF RESISTORS, AND SAID ADDITIONAL RESISTOR AND SAID LAST CONDENSER OF SAID FILTER CHAIN FORMING A NEGATIVE FEEDBACK PATH. 